Many types and sizes of powered mobile machines are used in construction, quarrying, and other industrial activities, including wheel-loaders, off-highway trucks, truck-type tractors, excavators, and other on-highway and off-highway machines. Such machines are typically propelled over the ground by any one of a number of available propulsion sources. Traditionally, such machines included a single power source, e.g., an internal combustion engine, coupled to wheels or tracks via a transmission. However, such an arrangement has the disadvantage that the engine must accelerate and decelerate between transmission ratios, and is thus generally not able to operate for extended periods of time within its optimal operating band. Moreover, the single power source must be sized so that it can accommodate everything from the smallest to the largest power loads that may potentially be faced by the machine. Given this, the power source is often greatly oversized for most loads placed upon it, leading to inefficient use of fuel.
As a partial solution to some of these problems, hybrid power sources began to see increased popularity. A traditional hybrid power source includes an engine, e.g., an IC engine, coupled to a generator. As the engine turns the generator, electrical energy is created and stored and, when needed, consumed. The electrical energy is used to drive one or more electric motors that provide the torque needed, via a transmission, to propel the machine. The ability to store electrical energy means that the engine can operate in a fuel-efficient steady state mode regardless of the varying power requirements of the machine. This traditional hybrid design is often referred to as a “series” hybrid design.
While the series hybrid design is fairly efficient and still enjoys widespread popularity, it does come with a number of drawbacks. For example, because the electrical motor or motors must supply all of the needed torque, these components must be somewhat overbuilt to anticipate all potential loads. This increases the cost and weight of the machine, and decreases the maximum efficiency of the machine.
In response to the various problems associated with single drive and series hybrid drive power schemes, the “parallel” hybrid drive power system was created. As the term is used herein, a parallel hybrid drive power system is one in which two different types of power source are used simultaneously to provide torque to the machine tracks or wheels. One of the power sources may also act as a source of power for the other power source. For example, an IC/electric parallel hybrid drive system uses both an IC engine and an electric motor to turn the wheels or tracks, but the engine also drives a generator that provides power for the electric motor.
Of particular interest for the present application is a type of IC/electric parallel hybrid power/transmission system known as an input split parallel path variable transmission. This system includes a transmission that receives power from the machine's engine and delivers power to wheels or tracks in order to propel the machine. One or more motor/generators linked into the transmission are also connected to energy storage devices, such as a battery, which accepts power from, and supplies power to, the motor/generators.
The transmission includes one or more planetary gear sets through which the engine and the one or more motor/generators are linked to one another and to the output shaft of the transmission. Selective engagement, disengagement, braking and clutching are used within the transmission to control the balance of power between the power sources and to define the relationship between the power sources and the transmission output in any given mode and/or gear.
The planetary gear system as configured in the aforementioned transmission is very useful in combining power from the multiple power sources. However, the efficiency of the transmission is less than ideal, and depends largely upon the degree of power mixing at any moment. For example, while the extensive use of both power sources is required during transient states, the overall system efficiency will be highest when the output power is derived exclusively from the engine.
When considering this background section, the disclosure and claims herein should not be limited by the deficiencies of the prior art. In other words, the solution of those deficiencies, while desirable, is not a critical limitation of any claim except where otherwise expressly noted in that claim. Moreover, while this background section is presented as a convenience to the reader who may not be of skill in this art, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is thus necessarily concise and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would refer to the demonstrated state of the printed art.